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Radiator cap leaking

My MK4 started to leak coolant out of the top of the radiator cap even though it was fitted with an overflow hose and tank (as provided by FFR). At first, I just noticed some minor staining on the SS cover shield covering the top of the radiator. I drained the coolant to replace it with fresh G5 coolant. I added a sacrificial zinc anode in the drain valve to reduce corrosion. But after I refilled the radiator and ran the engine up to operating temperature of 180 degrees, coolant started leaking out of the top of the cap.

I tested the cap with my SnapOn coolant tester and saw the cap would relieve at 15 psi vs the rated 16 psi. Normally not much to worry about but I had issues so I got another cap. It tested at 16.5 psi....so I give it a go. However, again it leaked. I made sure the recovery or overflow tank and hose wasn't blocked. But once the engine coolant temp hit 180, leaking started again.

It felt to me like the pressure seal wasn't making sufficient pressure against the top of the radiator fill adapter. I cleaned the sealing surface with a fine wire brush on my dremel tool. Both lower and upper sealing surfaces were now smooth as polished aluminum. Again it leaked like before. I the machined a .125" thick washer to sit on top if the bottom radiator adapter fill and used sealant to seal it to the adapter surface. I remounted the radiator cap and now it no longer leaked at 180. Even after driving around in 95 degree heat, no more leaks.

Has anyone else had any issues with the FFR radiator fill adapter ? It seems to me the neck was too deep to sit properly against the spring loaded cap's seal. Am I hallucinating ?

I had a similar issue and spent months trying to sort it out. Finally installed an 18 psi cap and the leak is gone. I have near 1500 miles with the new cap and all is well. I wish I knew the root cause. My setup is supposed to operate with a 14 psi cap.

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